IEEE802.11 which is a representative standard for wireless LAN (Local Area Network) systems adopts a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) scheme as a media access control method. According to the CSMA/CA scheme, transmission is performed after confirming through carrier sensing that a medium is continuously idle for a predetermined time or more to avoid data collision. The continuous wait time in that case is the sum of a minimum time and a wait time of a random length and prevents a plurality of communication apparatuses from simultaneously carrying out transmission a certain time after an immediately preceding communication.
Furthermore, to meet a demand for further enhancement of a transmission rate in a wireless LAN, various proposals of speed enhancement are presented in various techniques of the IEEE802.11n standard. As one such approach, a method of expanding a frequency band, which is a medium, is proposed. While existing IEEE802.11 wireless LAN systems (IEEE802.11a/b/g) carry out communication in a frequency band of 20 MHz per channel, IEEE802.11n extends a channel to a neighboring channel to make it possible to realize communication in a frequency band of 40 MHz corresponding to two channels including the neighboring channel (e.g., see IEEEP802.11n™/D2.00, February 2007).
The IEEE802.11n standard is said to aim at a throughput of over 100 Mbps, there may be demands for further speed enhancement in the future and the IEEE802.11 Study Group has actually started investigations into further speed enhancement. In this regard, one approach for further speed enhancement can be a method of further increasing the number of frequency channels used. As a transmission method in a case where the number of frequency channels used is increased, there can also be a method of realizing carrier sensing on a plurality of channels and carrying out transmission using an idle frequency channel as in the case of a method under study, for example, in a cognitive wireless system (e.g., see JP-A 2007-300421 (Kokai)).
Upon receiving a frame directed to the own terminal, an IEEE802.11 wireless LAN system needs to make a CRC confirmation on the frame and then send a return as to whether the frame has been successfully received after 16 μs, which is called “SIFS (Short Inter Frame Space)” using a response frame (e.g., ACK frame or CTS frame). Therefore, when frame transmission is carried out by applying such an idea like the one described in JP-A 2007-300421 (Kokai) to the IEEE802.11 wireless LAN system, the transmission terminal may simultaneously carry out frame transmission to the same terminal through a plurality of frequency channels, and furthermore the receiving terminal that has received the frame needs to carry out reception processing on the data of the plurality of frequency channels and then simultaneously return a response frame through all the channels of the successfully received frame after a lapse of SIFS. In this way, when such an idea like the one described in JP-A 2007-300421 (Kokai) is applied to the IEEE802.11 wireless LAN system, not only a transmission frame itself but also a response frame returned after SIFS is simultaneously transmitted through a plurality of frequency channels.
As described above, when frame transmission is carried out simultaneously using a plurality of frequency channels, there can be a problem that the reception characteristic at the receiving side terminal deteriorates as transmission power decreases at each channel. This will be explained below.
First, in the currently common mounting of an IEEE802.11-compliant terminal, the sum total of all transmission power is assumed to be constant, as the number of frequency channels used increases, the transmission power per channel generally becomes 1/(number of frequency channels) of that when transmission is carried out using only one channel. In this case, since the transmission power per channel decreases, a signal to noise ratio (SNR) deteriorates at a wireless communication terminal that has received each frame transmitted from each frequency channel with such transmission power compared to the case where transmission is carried out using one channel and a reception error rate increases. As also described above, a response frame after SIFS may also be transmitted using a plurality of frequency channels in an IEEE802.11 wireless LAN, but if a response frame containing reception success/failure information results in an error, more than necessary frame retransmissions may take place or the like and influences of deterioration of the error rate due to the deterioration of frame reception performance are considerable. According to the IEEE802.11n standard, the same frame is also transmitted by “Duplicate” using two channels (40 MHz), but in this case, the above described problem due to the deterioration of transmission power at each channel does not occur because of a combined gain obtained by combining the respective frames. This problem becomes more pronounced when frames are transmitted simultaneously through a plurality of frequency channels and the frames need to be individually handled without being combined on the respective frequency channels.
Furthermore, even when there are no restrictions on the above described mounting of the IEEE802.11-compliant terminal, since it is generally important to suppress power consumption of the wireless LAN terminal to a low level, even if the number of channels used is increased, it is hardly conceivable to multiply transmission power by the number of channels used and transmission power at each channel is considered to tend to be suppressed as the number of channels used increases. Should the number of channels used be increased, if transmission power per channel is not decreased, the reception performance does not deteriorate, but the transmission power at the entire wireless LAN terminal is multiplied by the number of channels used and power consumption thereby becomes a serious problem.
Furthermore, from the standpoint of power consumption, the terminal side which performs carrier sensing on a plurality of channels and carries out frame transmission through idle frequency channels may return a response frame indicating a reception success after SIFS through all frequency channels through which transmission has been carried out, and therefore the terminal side needs to wait for reception so as to be able to receive the response frame through all frequency channels, and power consumption through a reception filter or the like increases compared to the case where the terminal side needs to wait for only one channel. Furthermore, the processing accompanying transmission/reception through a plurality of channels is also a factor for an increase of power consumption compared to the case where transmission/reception is carried out through only one channel.